This invention relates to the fracturing of subterranean formations surrounding wellbores and more particularly, to the enhancement of fracturing by cooling of the formations.
The production rates of oil and gas wells are directly affected by the permeability of the producing formations adjacent the borehole. Various well known stimulation techniques are designed to increase the permeability of the formation at least near the borehole. Hydraulic fracturing has proved to be one of the most effective stimulation techniques since the fractures can be propagated great distances out into the formation.
The basic hydraulic fracturing technique involves the injection of a fluid into a formation at a pressure sufficiently above the ambient earth stresses to cause parting of the formation. Once a fracture has begun, it may typically be propagated at a pressure somewhat below the initial fracturing pressure. However, fractures are generally not controllable in terms of orientation or direction of travel. In deep wells, fractures tend to be vertical rather than horizontal but the exact orientation depends more on formation characteristics than on fracturing techniques. Since oil bearing zones tend to be thin layers, vertical fractures have a tendency to propagate above and/or below the oil bearing zone. Ideally, the fracture would be contained within the oil zone and extend laterally from the borehole as far as possible.
In some situations, formations other than the oil bearing zone of interest may be exposed to fracturing pressure. If the other zones have an initial fracturing pressure at or below that of the oil bearing zone, they will fracture first or at least in addition to the oil zone. Where such other zones cannot be physically isolated from the fracturing pressure, it is desirable to provide some other means of limiting the fractures to the desired zone.